If they're the (often yellow) ones on the posts, pointing towards the sign gantries, then they're 'spot' speed. Average speed cameras are usually one camera per lane, on separate posts, they'll be at regular intervals and at junction slips.

T&C's apply: this advice is to be considered little more than guesswork and should not be quoted as definitive.

I've seen plenty of average speed enforcement in the roadworks to convert motorways to "managed" or "smart". I don't think I've come across it on completed stretches.

Your point about lower speed limits on congested stretches increasing time separation interests me. I've noticed what feels like higher traffic density, with smaller distance separation, when lower limits were in force, although what is cause and what effect I cannot know. Next time I'm on the M6 I'll see what I notice about time gaps.

AIUI, the cameras in use on "smart" motorways are HADECS3. Highways Agency Digital Enforcement Camera System 3. They are usually grey and alongside the gantries. AFAIK, they are "always on" but I do not know exactly what enforcement they provide or to what tolerance.

kfae8959 wrote:Your point about lower speed limits on congested stretches increasing time separation interests me. I've noticed what feels like higher traffic density, with smaller distance separation, when lower limits were in force, although what is cause and what effect I cannot know. Next time I'm on the M6 I'll see what I notice about time gaps.

It's simple maffs innit? If the number of vehicles in a lane remains constant, then the distance between them must too. If they're traveling more slowly, then that distance will be travelled more slowly, so a greater time separation.

However, AFAIK the lower limits 'should' only be set when traffic volume increases, so that might result in what you report. Effect and cause, tomaydo tomato.

My Observations Normal motorwaysLane 1 mainly convoys of trucks usually 0.25 second gap whilst they slipstream driving on the limiter at 56 MPHlane 2 1 car driving at 45 miles per hour in from 30 seconds behind 20 cars all less than a second between them Guys at the back just caught up with them at 90 mph and had to brake hard as he had not noticed them He will stay behind in lane two until all the cars in front move out of his way then remaining in lane 2 he will accelerate to 90 again to catch the next convoy.Lane 3 all doing 80 but wanting to do more less than 0.5 second gap

(Not so) Smart motorways and average speed camera motorwaysLane 1 mainly convoys of trucks usually 0.25 second gap whilst they slipstream driving on the limiter at 56 MPHlane 2 1 car driving at 45 miles per hour in from 30 seconds behind 20 cars all less than a second between them Guys at the back just caught up with them at 90 mph but slowing down for the speed cameras and had to brake hard as he had not noticed the queue in front of him. He will stay behind in lane two until all the cars in front move out of his way then remaining in lane 2 he will accelerate to 90 again to catch the next convoy.Already has 18 points on his license Lane 3 all doing 5 mph more than the posted limit but wanting to do more less than 0.5 second gap